1. Field of the Invention
The invention relates to a method of thermally treating polyester pellets, to obtain a partial crystallization, and apparatus for achieving same. More particularly, the invention relates to a method and apparatus using an underwater granulator.
2. Description of the Prior Art
Aromatic polyesters or co-polyesters, particularly polyethylene terephthalate and its co-polymers, as well as polytrimethylene terephthalate and polyethylene naphthalate, formed into granulates, are the base material for products such as foils, packaging, as well as barrels or containers. All of these polyesters and co-polyesters shall be designated generally as “polyesters” in the description of the present invention.
A conventional method for thermally treating polyester pellets is disclosed in the subsequently published Application Publication WO 2005/044901 A1 (BKG). This method teaches removing water very quickly from the pellets that have just been produced, for example, in an “underwater heat-rejection granulation system,” by using the intrinsic heat of the pellets for drying and crystallization. The pellets are transported over a vibrating or swinging conveyor immediately after the water is removed, in order to prevent the pellets from sticking to each other and, after sufficient dwell time, carried or fed to a filling station or a solid-phase poly condenser. With this method, the crystallization process occurs from the inside to the outside of the pellet, which achieves a more even crystallization across the diameter of the pellet or granulate. Thus, this method achieves crystallization exclusively by using the intrinsic heat generated in the liquid state of the polymer. This means that no externally supplied heat is applied to the pellets between granulation and the subsequent filling station or post-processing station. This distinguishes the method according to WO 2005/044901 A1 essentially from conventional methods.
A method of manufacturing polyethylene terephthalate granulates is described in the publication GB 1 250 690 A. According to this method, the base material is produced as in the conventional liquid condensation and fed to an underwater granulator as an approximately 280 degree C. hot polyester liquid. The strand of hot synthetic material that is pressed through a jet is captured and cooled by a water spray coming out of a ring jet. This strand is then fed through an underwater cooling distance to a cutting device. After passing through the cutting device, the granulate material traverses a sieve, by means of which water is separated from solid material. The water is pumped across a cooler in a closed circuit. The wet granulates, after they have been dried, can be put through thermal post-condensation. These granulates, particularly when treated in a thermal post-condensation at greater than 200 degree C., are suitable in their solid state for injection molding.
Essential with this process is that the product that is produced in the strand granulation process, i.e., an underwater cold-rejection granulating system, which is consequently a substantially cooled product, has to be re-heated again, if it is to undergo any further thermal treatment, which includes treatment for the partial crystallization. This requires a substantial amount of energy, which is lost with the preceding cooling method.
U.S. Pat. No. 5,540,868 teaches how to produce crystallized pellets from amorphous polyester, using various granulating methods. The amorphous polyester pre-product has to be heated to temperature greater than 70 degrees C., in order to initiate the crystallization process. Amorphous polyester, however, has a disadvantage in that, when heated to temperatures greater than 70 degrees C., it has a sticky surface. In order to prevent the amorphous polyester from sticking or forming clumps at crystallization temperatures of greater than 70 degrees C., the pre-product has to be in the form of granulates, which can then be held in motion in a cyclone bed reactor by means of streams of hot gas, until the surface has crystallized out enough, so that the pre-product granulates do not stick to each other.
Amorphous polyester is transparent; the crystalline phase, however, clearly shows a white coloration. Normally, the crystallization process of the pre-product is combined with the additional reinforced poly condensation, which is typically carried out at temperatures between 200 and 230 degrees C. in a cyclone bed reactor. This is done to overcome the stickiness of the amorphous polyester. To achieve this, the reactor is operated first at an optimal crystallization temperature of approximately 150 degrees C. for several hours, to overcome the stickiness. Subsequently, the pellets or granulates are condensed for additional hours to higher chain lengths at temperatures between 200 and 230 degrees C.
The method according to the invention is not described in the subsequently published German Application Publication DE 10 2004 050 356 A1. Rather, in the citation, claim 11 recites a post-processing granulating apparatus that includes a heat-insulating container.
Language is provided, shown below, at the end of paragraph 0010 on page 3 that describes which method step is to be performed with this heat-insulating container.
“The hot granulate material can also be stored in a heat-retaining condition, for example, in a heat-insulating container, in order to complete the desired crystallization process.”
The granulate material is not transported through the heat-insulating container, but rather, stored in the container, with the hope that this storage will lead to crystallization of the granulates, without the granulates baking and sticking to each other. Furthermore, the granulates should be held in a heat-retaining condition, that is, they should not cool down or heat up.
Aside from the fact that storage of hot granulates leaving the dryer, without a motion device or without a motion of the granulate material itself, will only result in the granulates sticking to each other, the teaching of this reference does not teach the method of the present application.